Gas generant compositions

ABSTRACT

A gas generating composition  12  containing a polyvinylazole is contained within an exemplary gas generator  10 . A gas generating system  200  incorporates the polyvinylazole therein. A vehicle occupant protection system  180  incorporates the gas generating system  200.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 60/624,288 filed on Nov. 1, 2004.

TECHNICAL FIELD

The present invention relates generally to gas generating systems, and to gas generant compositions employed in gas generator devices for automotive restraint systems, for example.

BACKGROUND OF THE INVENTION

The present invention relates to nontoxic gas generating compositions that upon combustion rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to thermally stable nonazide gas generants having not only acceptable burn rates, but that also, upon combustion, exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures.

The evolution from azide-based gas generants to nonazide gas generants is well-documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and 5,035,757, the discussions of which are hereby incorporated by reference.

In addition to a fuel constituent, pyrotechnic nonazide gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.

One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. When employed in a vehicle occupant protection system, the solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate.

The use of phase stabilized ammonium nitrate as an oxidizer, for example, is desirable because it generates abundant nontoxic gases and minimal solids upon combustion. To be useful, however, gas generants for automotive applications must be thermally stable when aged for 400 hours or more at 107 degrees C. The compositions must also retain structural integrity when cycled between −40 degrees C. and 107 degrees C. Further, gas generant compositions incorporating phase stabilized or pure ammonium nitrate sometimes exhibit poor thermal stability, and produce unacceptably high levels of toxic gases, CO and NO_(x) for example, depending on the composition of the associated additives such as plasticizers and binders. Furthermore, recent revisions in U.S. car requirements require relatively minimal amounts of ammonia in the effluent gases.

Yet another problem that must be addressed is that the U.S. Department of Transportation (DOT) regulations require “cap testing” for gas generants. Because of the sensitivity to detonation of fuels known for their use in conjunction with ammonium nitrate, triaminoguanidine nitrate for example, many propellants incorporating ammonium nitrate do not pass the cap test unless shaped into large disks, which in turn reduces design flexibility of the inflator.

Yet another concern includes slower cold start ignitions of typical smokeless gas generant compositions, that is gas generant compositions that result in less than 10% of solid combustion products.

Accordingly, ongoing efforts in the design of automotive gas generating systems, for example, include other initiatives that desirably produce more gas and less solids without the drawbacks mentioned above.

SUMMARY OF THE INVENTION

The above-referenced concerns are resolved by gas generating systems including a gas generant composition containing an extrudable polyvinylazole fuel such as a polyvinyltetrazole, polyvinyltriazole, or polyvinyidiazole. Preferred oxidizers include nonmetal oxidizers such as ammonium nitrate and ammonium perchlorate. Other oxidizers include alkali and alkaline earth metal nitrates.

The fuel is selected from the group of polyvinyltetrazoles, polyvinyltriazoles, polyvinyldiazoles, and polyvinylfurazans, and mixtures thereof. An exemplary group of fuels includes polymeric tetrazoles, triazoles, and oxadiazoles (furazans), having functional groups on the azole pendants. Preferred vinyl tetrazoles include 5-Amino-1-vinyltetrazole and poly(5-vinyltetrazole), both exhibiting self-propagating thermolysis or thermal decomposition. Other fuels include poly(2-methyl-5-vinyl) tetrazole, poly(1-vinyl) tetrazole, poly(3-vinyl) 1,2,5-oxadiazole, and poly(3-vinyl) 1,2,4-triazole. These and other possible fuels are structurally illustrated in the figures included herewith. As such, the polyvinylazoles may exhibit pendant aromaticity, or, aromatic character within the polymer backbone, depending on the design criteria of the gas generant composition, and depending on the starting reagents in the synthesis of the polyvinylazole. In the present invention, the fuel may typically constitute about 5-40% by weight of the gas generant composition.

An oxidizer is preferably selected from the group of nonmetal, and alkali and alkaline earth metal nitrates, and mixtures thereof. Nonmetal nitrates include ammonium nitrate and phase stabilized ammonium nitrate, stabilized as known in the art. Alkali and alkaline earth metal nitrates include potassium nitrate and strontium nitrate. Other oxidizers known for their utility in air bag gas generating compositions are also contemplated. In the present invention, the oxidizer may typically constitute about 60-95% by weight of the gas generant composition.

Other gas generant constituents known for their utility within vehicle occupant protection systems, and within gas generant compositions typically contained therein, may be employed in functionally effective amounts in the compositions of the present invention. These include, but are not limited to, coolants, slag formers, and ballistic modifiers known in the art.

In sum, the present invention includes gas generant compositions that maximize gas combustion products and minimize solid combustion products while retaining other design requirements such as thermal stability. These and other advantages will be apparent upon a review of the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view showing the general structure of an inflator in accordance with the present invention;

FIG. 2 is a schematic representation of an exemplary vehicle occupant restraint system containing a gas generant composition in accordance with the present invention.

FIG. 3 and FIG. 4 are graphical representations of respective burn rates compared to combustion pressure of gas generant compositions.

FIG. 5 and FIG. 6 are graphical representations indicating combustion profiles of the same gas generant before and after aging for 400 hours at 107 C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention generally relates to gas generant compositions for inflators of occupant restraint systems. In accordance with the present invention, a pyrotechnic composition includes extrudable fuels such as polyvinyltetrazoles (PVT) for use within a gas generating system, such as that exemplified by a high gas yield automotive airbag propellant in a vehicle occupant protection system. The fuel also functions as a binder. Preferred oxidizers include nonmetal oxidizers such as ammonium nitrate and ammonium perchlorate. Other oxidizers include alkali and alkaline earth metal nitrates.

The fuel is selected from the group of polyvinyltetrazoles, polyvinyltriazoles, polyvinyidiazoles or polyvinylfurazans, and mixtures thereof. An exemplary group of fuels includes polymeric tetrazoles, triazoles, and oxadiazoles (furazans), optionally having functional groups on the azole pendants. Vinyl tetrazoles include 5-Amino-1-vinyltetrazole and poly(5-vinyltetrazole), both exhibiting self-propagating thermolysis or thermal decomposition. Other fuels include poly(2-methyl-5-vinyl) tetrazole, poly(1-vinyl) tetrazole, poly(3-vinyl) 1,2,5-oxadiazole, and poly(3-vinyl) 1,2,4-triazole. These and other possible fuels are exemplified by, but not limited to, the structures shown below.

It has been discovered that an unexpected additional benefit with the inclusion of the present fuels is that compositions resulting in difficult cold-start ignitions that necessitate more powerful ignition trains and boosters, are avoided. Poly(5-amino-1-vinyl) tetrazole, for example, is believed to have no endothermic process before exothermic decomposition begins. Therefore, the heat-consuming step normally attendant prior to the energy releasing steps of combustion (that acts as an energy barrier), is apparently not present in the present compositions. It is believed that other polymeric azoles functioning as fuels in the present invention have the same benefit. In the present invention, the polyvinylazole fuel or the total fuel component of the gas generant composition may typically constitute about 5-40% by weight of the gas generant composition.

An oxidizer is preferably selected from the group of nonmetal, and alkali and alkaline earth metal nitrates, and mixtures thereof. Nonmetal nitrates include phase stabilized ammonium nitrate, stabilized as known in the art for example. Alkali and alkaline earth metal nitrates include potassium nitrate and strontium nitrate. It has been found that in accordance with the present invention, compositions containing phase stabilized ammonium nitrate exhibit sufficient thermal stability, in contrast to many other known compositions containing unstabilized ammonium nitrate and/or phase stabilized ammonium nitrate. Other oxidizers known for their utility in air bag gas generating compositions are also contemplated. It must be appreciated, however, that the oxidizers of the present invention provide an overall oxygen balance within the combustion reaction to minimize the production of carbon monoxide and/or nitrogen oxides. The oxygen balance of the present invention as determined by the oxidizer provided ranges from −4.0% to +4.0%. It will be appreciated that in gun propellants for example, the amount of oxygen balance purposefully results in carbon monoxide upon combustion of the respective gun propellant thereby providing the required thrust with the lowest possible molecular weight gases. In the present invention, the oxidizer or the total oxidizer component may typically constitute about 60-95% by weight of the gas generant composition.

Other gas generant constituents known for their utility in air bag gas generant compositions may be employed in functionally effective amounts in the compositions of the present invention. These include, but are not limited to, coolants, slag formers, and ballistic modifiers known in the art.

The gas generant constituents of the present invention are supplied by suppliers known in the art and are preferably blended by a wet method. Typical or known suppliers include Aldrich or Fisher Chemical companies. A solvent chosen with regard to the group(s) substituted on the polymeric fuel is heated to a temperature sufficient to dissolve the fuel but below boiling, for example just below 100° C., but low enough to prevent autoignition of any of the constituents as they are added and then later precipitate. Hydrophilic groups, for example, may be more efficiently dissolved by the use of water as a solvent. Other groups may be more efficiently dissolved in an acidic solution, nitric acid for example. Other solvents include alcohols and plasticizers such as polyethylene glycol. Once a suitable solvent is chosen and heated, the fuel is slowly added and dissolved. The oxidizer is then slowly added and also at least partially dissolved. Any other desirable constituents are likewise dissolved. The solution is heated and continually stirred. As the solvent is evaporated off over time, the fuel and oxidizer, and any other constituents, are co-precipitated in a homogeneous solid solution. The precipitate is removed from the heat once the solvent has been at least substantially volatilized, but more preferably completely volatilized. The composition may then be extruded into pellets or any other useful shape.

The polymeric fuels may be manufactured by known processes. For example, vinylation of a tetrazole with vinyl acetate, followed by polymerization is described in Vereshchagin, et al., J. Org. Chem. USSR (Engl. Transl.) 22(9), 1777-83, (1987). The synthesis of various vinyltetrazoles is also described in Russian Chemical Reviews 72(2), pages 143-164 (2003), herein incorporated by reference. The methyl-group of the starting tetrazole can be exchanged for an amino group. The vinyltetrazoles are then polymerized using a common polymerization initiator such as azoisobutyronitrile (AIBN). It is believed that similar vinylation of furazans and triazoles will also yield the polyvinyldiazoles and polyvinyltriazoles of the present invention. Exemplary reactions given below illustrate how various polyvinyidiazoles, polyvinyltriazoles and polyvinyltetrazoles may be formed. Reactions 1, 2, and 3 illustrate how polyvinyldiazoles, polyvinyltriazoles, and polyvinyltetrazoles, respectively, may be formed.

Reaction 1: This synthesis illustrates the production of poly(vinyl-1,2,5-oxadiazole) and exemplifies or blueprints a general method of forming polyvinyldiazoles.

Reaction 2: This synthesis illustrates the production of an ionic polymer version of poly(vinyl-1,2,4-triazole) and exemplifies or blueprints a method of forming other polyvinyltriazoles.

where R=CH₃, NH₂, etc. Reaction 3: This synthesis illustrates the production of a substituted polyvinyltetrazole and exemplifies or blueprints a method of forming other polyvinyltetrazoles.

A generic polyvinylazole, or a structure that generically represents the polyvinyltetrazoles, polyvinyltriazoles, and polyvinyidiazoles of the present invention, may be represented by an aromatic ring having five cites that contain,

Stated another way, the aromatic ring will contain from zero to a single oxygen atom, will contain at least two nitrogen atoms, and will contain at least one carbon atom. More preferably, a gas generant composition of the present invention will contain a polymeric azole and phase stabilized ammonium nitrate. The advantages are high gas yield and low solids production, a high energy fuel/binder, and a low-cost oxidizer thereby obviating the need for filtration of the gas given that little if any solids are produced upon combustion. The compositions of the present invention may be extruded given the pliant nature of the polymeric fuels.

The gas generant compositions of the present invention may also contain a secondary fuel formed from amine salts of tetrazoles and triazoles. These are described and exemplified in co-owned U.S. Pat. Nos. 5,872,329, 6,074,502, 6,210,505, and 6,306,232, each herein incorporated by reference. The total weight percent of both the first and second fuels, or the fuel component of the present compositions, is about 10 to 40 weight % of the total gas generant composition. As shown in the data presented in FIGS. 3 and 4, the use of a secondary fuel provides enhanced burn rates as pressure increases.

More specifically, nonmetal salts of tetrazoles and triazoles include in particular, amine, amino, and amide salts of tetrazole and triazole selected from the group including monoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.1GAD), diguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.2GAD), monoaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.1AGAD), diaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.2AGAD), monohydrazinium salt of 5,5′-Bis-1H-tetrazole (BHT.1HH), dihydrazinium salt of 5,5′-Bis-1H-tetrazole (BHT.2HH), monoammonium salt of 5,5′-bis-1H-tetrazole (BHT.1NH₃), diammonium salt of 5,5′-bis-1H-tetrazole (BHT.2NH₃), mono-3-amino-1,2,4-triazolium salt of 5,5′-bis-1H-tetrazole (BHT.1ATAZ), di-3-amino-1,2,4-triazolium salt of 5,5′-bis-1H-tetrazole (BHT.2ATAZ), and diguanidinium salt of 5,5′-Azobis-1H-tetrazole (ABHT.2GAD).

Amine salts of triazoles include monoammonium salt of 3-nitro-1,2,4-triazole (NTA.1NH₃), monoguanidinium salt of 3-nitro-1,2,4-triazole (NTA.1GAD), diammonium salt of dinitrobitriazole (DNBTR.2NH₃), diguanidinium salt of dinitrobitriazole (DNBTR.2GAD), and monoammonium salt of 3,5-dinitro-1,2,4-triazole (DNTR.1NH₃).

A generic nonmetal salt of tetrazole as shown in Formula I includes a cationic nitrogen containing component, Z, and an anionic component comprising a tetrazole ring and an R group substituted on the 5-position of the tetrazole ring. A generic nonmetal salt of triazole as shown in Formula II includes a cationic nitrogen containing component, Z, and an anionic component comprising a triazole ring and two R groups substituted on the 3- and 5-positions of the triazole ring, wherein R₁ may or may not be structurally synonymous with R₂. An R component is selected from a group including hydrogen or any nitrogen-containing compound such as an amino, nitro, nitramino, or a tetrazolyl or triazolyl group as shown in Formula I or II, respectively, substituted directly or via amine, diazo, or triazo groups. The compound Z is substituted at the 1-position of either formula, and is formed from a member of the group comprising amines, aminos, and amides including ammonia, carbohydrazide, oxamic hydrazide, and hydrazine; guanidine compounds such as guanidine, aminoguanidine, diaminoguanidine, triaminoguanidine, dicyandiamide and nitroguanidine; nitrogen substituted carbonyl compounds or amides such as urea, oxamide, bis-(carbonamide) amine, azodicarbonamide, and hydrazodicarbonamide; and, amino azoles such as 3-amino-1,2,4-triazole, 3-amino-5-nitro-1,2,4-triazole, 5-aminotetrazole, 3-nitramino-1,2,4-triazole, 5-nitraminotetrazole, and melamine.

Example 1

A gas generant composition of the present invention is formed by first synthesizing a polyvinyltetrazole. A generic substituted tetrazole and vinyl acetate are combined to vinylate the tetrazole. The vinylated tetrazole is added to a molar equivalent of mercury acetate and boron trifluoride-etherate for polymerization thereof. The resulting products may then be separated by oil distillation for example. The polyvinyltetrazoles illustrated in the drawings may be formed in the same generic way. Reaction 3 exemplifies the process described above.

Example 2

A gas generant composition of the present invention is formed by first synthesizing a polyvinyltriazole. A generic substituted triazole metal or nonmetal salt is added to a molar equivalent amount of a free radical brominating reagent such as n-bromo-succinamide and to a benzoyl-peroxide free radical initiator to form a brominated triazole. The brominated triazole is then added to triphenyl phosphine to form a Wittig salt group on the substituted triazole salt. The triazole salt is then added to a metal or nonmetal organic or inorganic base, and also to formaldehyde to form a vinylated triazole salt. The vinylated triazole salt is next added to a free radical polymerization reagent such as azoisobutyronitrile and a catalytic amount of a cationic polymerizer or Ziegler-Natta catalyst such as a metal or titanium complex. Reaction 2 exemplifies the process described above, wherein the synthesis of poly(vinyl-1,2,4-triazole) is shown.

Example 3

A gas generant composition of the present invention is formed by first synthesizing a polyvinyldiazole. An alkenol containing two —OH groups is added to acetic anhydride to form a substituted diazole. The substituted diazole is then added to a molar equivalent amount of a free radical brominating reagent such as n-bromo-succinamide and to a free radical initiator such as benzoyl-peroxide to form a brominated diazole. The substituted diazole is then added to triphenyl phosphine to form a Wittig salt group on the substituted diazole salt. The diazole salt is then added to a metal or nonmetal organic or inorganic base, and also to formaldehyde to form a vinylated diazole salt. The vinylated diazole salt is next added to a free radical polymerization reagent such as azoisobutyronitrile and a catalytic amount of a cationic polymerizer or Ziegler-Natta reagent such as a metal complex. Reaction 1 exemplifies the process described above wherein the synthesis of poly(vinyl-1,2,5-oxadiazole) is shown.

Examples 4-9

Examples 4-9 are tabulated below and provide a comparative view of the different types and amounts of gas produced with regard to several known gas generant compositions and a gas generant formed in accordance with the present invention. Example 4 is a representative gas generant composition formed from 5-aminotetrazole and strontium nitrate, in accordance with U.S. Pat. No. 5,035,757 herein incorporated by reference. Example 5 is a representative gas generant composition formed from an amine salt of tetrazole such as diammonium salt of 5,5′-bi-1H-tetrazole, phase stabilized ammonium nitrate, strontium nitrate, and clay in accordance with U.S. Pat. No. 6,210,505 herein incorporated by reference. Example 6 is a representative gas generant composition formed from an amine salt of tetrazole such as diammonium salt of 5,5′-bi-1H-tetrazole and phase stabilized ammonium nitrate in accordance with U.S. Pat. No. 5,872,329 herein incorporated by reference. Example 7 is a representative gas generant composition formed from ammonium nitramine tetrazole and phase stabilized ammonium nitrate in accordance with U.S. Pat. No. 5,872,329 herein incorporated by reference. Example 8 is a representative gas generant composition formed from ammonium nitramine tetrazole, phase stabilized ammonium nitrate, and a slag former in accordance with U.S. Pat. No. 5,872,329 herein incorporated by reference. Example 9 is a representative composition formed in accordance with the present invention containing ammonium polyvinyl tetrazole at 15% by weight and phase stabilized ammonium nitrate at about 85% by weight (ammonium nitrate coprecipitated with 10% potassium nitrate).

Table 1 details the relative amounts produced (ppm) of carbon monoxide (CO), ammonia (NH3), nitrogen monoxide (NO), and nitrogen dioxide (NO2) with regard to each example and the amount of gas generant in grams (g). All examples were combusted in a gas generator of substantially the same design.

TABLE 1 Example g P_(c) CO NH3 NO NO2 4 45 15 125 10 49 9 5 25 36 109 65 29 4 6 25 29 111 29 37 5 7 25 36 62 10 28 3 8 25 37 98 35 33 4 9 25 34 129 4 28 4

The data collected indicates that the composition of Example 9, formed in accordance with the present invention, results in far less ammonia than the other examples, well below the industry standard of 35 ppm. It has been discovered that compositions of the present invention result in substantially less amounts of ammonia as compared to other known gas generants. In certain known gas generant compositions, it is often difficult to reduce the total amount of ammonia produced upon combustion, even though other performance criteria remain favorable.

Examples 10-14

Theoretical examples 10-14 are tabulated below and provide a comparative view of the different amounts and types of gas produced with regard to several gas generant compositions formed in accordance with the present invention. All phase stabilized ammonium nitrate (PSAN10) referred to in Table 2 has been stabilized with 10% by weight potassium nitrate of the total PSAN. All examples employ ammonium poly(C-vinyltetrazole) (APV) as the primary fuel. Certain examples employ nonmetal diammonium salt of 5,5′-Bis-1H-tetrazole (BHT.2NH3) as a secondary fuel. All examples reflect results generated by combustion of the gas generant constituents (propellant composition) within a similarly designed inflator or gas generator with equivalent heat sink design.

TABLE 2 Constituents Flame Exhaust Gas Combustion Ex. (wt % of 100 g) Temp. (K) Temp. (K) Products (mol) 10 15% APV 2222 857 2.25 H2O 85% PSAN10 1.33 N2 0.39 CO2 11 16% APV 2039 890 2.0 H2O 69% PSAN10 1.2 N2 10% Strontium Nitrate 0.37 CO2 05% Clay 12 22% APV 2054 1225 0.64 H2O 73% Strontium Nitrate 0.83 N2 05% Clay 0.52 CO2 13 08% APV 2036 874 1.86 H2O 64.60% PSAN10 1.34 N2 10% Strontium Nitrate 0.35 CO2 05% Clay 12.40% BHT•2NH3 14 08% APV 2206 835 2.20 H2O 80.60% PSAN10 1.45 N2 11.40% BHT•2NH3 0.34 CO2

Example 10 has been found to be thermally stable at 107 degrees Celsius for 400 hours with only a 0.5% mass loss. Accordingly, Example 10 exemplifies the unexpected thermal stability of gas generant compositions of the present invention, particularly those incorporating a polyvinylazole as defined herein and phase stabilized ammonium nitrate (stabilized with 10% potassium nitrate). It should be emphasized that other phase stabilizers are also contemplated as known or recognized in the art.

Examples 11 through 13 exemplify the use of a polyvinylazole with metallic oxidizers. In certain applications, the use of a metallic oxidizer may be desired for optimization of ignitability, burn rate exponent, gas generant burn rate, and other design criteria. The examples illustrate that the more metallic oxidizer is used the less mols of gas produced upon combustion.

In contrast, Examples 10 and 14 illustrate that molar amounts of gas combustion products are maximized when nonmetal gas generant constituents are employed. Accordingly, preferred gas generant compositions of the present invention contain at least one polyvinylazole as a fuel component and a nonmetal oxidizer as an oxidizer component.

Finally, with regard to Example 14, it has been found that the gas generant burn rate may be enhanced by adding another nonmetal fuel, BHT.2NH3, to APV and PSAN10, thereby optimizing the combustion profile of the gas generant composition. The burn rate of Example 14 is recorded at 1.2 inches per second at 5500 psi. It can be concluded therefore, that the addition of nonmetal amine salts of tetrazoles and/or nonmetal amine salts of triazoles as described in U.S. Pat. No. 5,872,329 may be advantageous with regard to burn rate and gas generation. Furthermore, the pliant nature of the APV provides extrudability of the propellant composition. As shown in FIGS. 3 and 4, the addition of a secondary fuel as indicated above, results in enhanced burn rates as the combustion pressure is elevated. FIG. 3 illustrates the burn rates over pressure of a formulation containing only polyvinyltetrazole and phase stabilized ammonium nitrate. In contrast, FIG. 4 illustrates the burn rates over pressure of a similar composition of FIG. 3 containing polyvinyltetrazole and phase stabilized ammonium nitrate, but with the addition of di-ammonium BHT, in accordance with the percent weight ranges provided herein. It can be concluded that the addition of a secondary fuel as described herein results in a significant advantage with regard to burn rate. The practical effect is repeatability of performance and enhanced ballistic properties of gas generants formed accordingly.

Examples 15 and 16

Examples 15 and 16 exemplify the cold start advantage of gas generant compositions containing a polyvinylazole. As indicated by differential scanning calorimetry (DSC), typical smokeless or nonmetal compositions may exhibit an endothermic trend prior to exothermic combustion. As a result, relatively greater amounts of energy must be available to ignite the gas generant and sustain combustion of the same. Oftentimes, a more aggressive ignition train, to include an aggressive booster composition perhaps, is required to attain the energy level necessary to ignite the gas generant and sustain combustion. Example 15 pertains to a composition containing 65% PSAN10 and about 35% BHT.2NH3. As indicated by DSC testing, an endotherm is maximized at 253.12 degrees Celsius, thereby representing a recorded loss of about 508.30 joules/gram of gas generant. In comparison, Example 16 pertains to a composition containing about 15% poly(C-vinyltetrazole) and about 85% PSAN10. Most unexpectedly, there is no endothermic process as indicated by DSC and accordingly, combustion proceeds in an uninhibited manner. As a result, less energy is required to combust the gas generant composition thereby reducing the ignition train or ignition and booster requirements.

Example 17

Another advantage of the present invention is illustrated in FIGS. 5 and 6. FIG. 5 illustrates baseline, or pre-aged combustion of a gas generating composition made in accordance with the present invention, within a state-of-the-art gas generator. Specifically, the gas generating composition contained 80.6% PSAN, 8.0% PVT, and 11.4% BHT-2NH3. FIG. 6, on the other hand, illustrates post-aged combustion of the same gas generating composition within the same gas generator. The curves indicated in FIGS. 5 and 6 correlate to “hot” or +85 C, “ambient” or 23 C, and “cold” or −40 C deployments of standard driver side inflators within a 60 liter ballistics tank. As shown, there is an insignificant combustion profile difference between the operation of the pre-aged and post-aged gas generants, when deployed at the three various temperatures. It will be appreciated that USCAR specifications for “accelerated aging” are 400 hours at 107 C. Typically, many car manufacturers worldwide require similar accelerated aging criteria.

In yet another aspect of the invention, the present compositions may be employed within a gas generating system. For example, as schematically shown in FIG. 2, a vehicle occupant protection system made in a known way contains crash sensors in electrical communication with an airbag inflator in the steering wheel, and also with a seatbelt assembly. The gas generating compositions of the present invention may be employed in both subassemblies within the broader vehicle occupant protection system or gas generating system. More specifically, each gas generator employed in the automotive gas generating system may contain a gas generating composition as described herein.

Exemplary gas generant compositions include poly(c-vinyltetrazole) as a fuel/binder at 5-40%, and more preferably 5-20%. Of course the weight percents provided for each of the constituents given below may be applicable to any polyvinylazole incorporated within the present invention. Various poly(c-vinyltetrazoles) may be made by the water-based method of manufacture described below. Or, the fuel/binder poly(c-vinyltetrazoles) may be formed as described in U.S. Pat. No. 3,096,312, herein incorporated by reference, for example. The polyvinyltetrazole salts may be either metallic or nonmetallic in character, whereby nonmetallic salts are preferred, but not limited thereby, due to the production of more gas and less solids.

The functionalization of the polymer is exemplified by the diagram of the PVT acid form given above. The pseudo-allylic carbon at position 1 of the backbone could possibly be functionalized with nitro or nitroso groups. This position could be functionalized with anything that makes the oxygen balance less negative or adds a source of fuel. This position may also be functionalized with any group to increase its materials properties (i.e. plasticity) such as a vinyl group for crosslinking (curing).

The nitrogen at position 2 is acidic with a preferable pKa of between 3-5. The attached hydrogen can therefore by removed with a mild base, leaving an aromatic tetrazole ring anion. Any suitable base can be used to make a water-soluble salt of the polymer as given in the water-based method described below. This position could also be functionalized with any group, to again, make the oxygen balance of the system less negative or to add more fuel. This position could also be functionalized with any group to impart a desired materials property to the system, such as a vinyl group for crosslinking.

A primary oxidizer is selected from the group including nitrate salts such as phase-stabilized ammonium nitrate, alkali and alkaline earth metal potassium nitrate, strontium nitrate; nitrite salts such as potassium nitrite; chlorate salts such as potassium chlorate; perchlorate salts such as potassium perchlorate; oxides such as iron oxide and copper oxide; basic nitrate salts such as basic copper nitrate and basic iron nitrate; and mixtures or combinations thereof. The oxidizer component of the present invention typically constitutes 60-95% of the total composition.

A secondary fuel may be selected from amine salts of tetrazoles and triazoles as given above, and may also be selected from azoles such as tetrazoles, triazoles, and furazans, such as 5-aminotetrazole; metal salts of azoles such as potassium 5-aminotetrazole; nonmetal salts of azoles such as mono- or di-ammonium salt of 5,5′-bis-1H-tetrazole; nitrate salts of azoles such as 5-aminotetrazole nitrate; nitramine derivatives of azoles such as 5-nitraminotetrazole; metal salts of nitramine derivative of azoles such as dipotassium 5-nitraminotetrazole; nonmetal salts of nitramine derivatives of azoles such as mono- or di-ammonium 5-nitraminotetrazole; and, guanidines such as dicyandiamide; salts of guanidines such as guanidine nitrate; nitro derivatives of guanidines such as nitroguanidine; azoamides such as azodicarbonamide; nitrate salts of azoamides such as azodicarbonamidine dinitrate; and mixtures and combinations thereof. The secondary fuel in combination with the primary polyvinylazole fuel typically constitutes about 5-40% by weight of the total composition as a fuel component thereof. The primary fuel constitutes about 13-100% of the fuel component (or about 5-40% by weight of the total composition), and the secondary fuel constitutes about 0-87% (or about 0-35% by weight of the total composition).

A slag former may be selected from the group including silicon compounds such as elemental silicon, and silicon dioxide; silicones such as polydimethylsiloxane; silicates such as potassium silicates; natural minerals such as talc, mica, and clay; and mixtures thereof. The slag former preferably constitutes 0-10% and more preferably constitutes 0-5% of the total composition.

Extrusion aides may be selected from the group including talc, graphite, borazine [(BN)₃], boron nitride, fumed silica, and fumed alumina. The slag former preferably constitutes 0-10% and more preferably constitutes 0-5% of the total composition.

The compositions may be dry or wet mixed using methods known in the art. The various constituents are generally provided in particulate form and mixed to form a uniform mixture with the other gas generant constituents. The mixture is then palletized or formed into other useful shapes in a safe manner known in the art. Preferred gas generant compositions include 60-95% by weight of phase-stabilized ammonium nitrate (10-15% KNO3) and 5-40% by weight of a nonmetal poly(c-vinyltetrazole). For example, a gas generant composition may include 85% by weight of phase-stabilized ammonium nitrate and 15% by weight of ammonium poly(c-vinyltetrazole). An even more preferred gas generant composition includes about 8% ammonium poly(c-vinyltetrazole), about 11% diammonium salt of 5,5′-bis-1H-tetrazole, and about 81% phase-stabilized ammonium nitrate.

It should be noted that all percents given herein are weight percents based on the total weight of the gas generant composition. The chemicals described herein may be supplied by companies such as Aldrich Chemical Company and Polysciences, Inc. for example.

As shown in FIG. 1, an exemplary inflator incorporates a dual chamber design to tailor the force of deployment an associated airbag. In general, an inflator containing a primary gas generant 12 and an autoignition composition 14 formed as described herein, may be manufactured as known in the art. U.S. Pat. Nos. 6,422,601, 6,805,377, 6,659,500, 6,749,219, and 6,752,421 exemplify typical airbag inflator designs and are each incorporated herein by reference in their entirety.

Referring now to FIG. 2, the exemplary inflator 10 described above may also be incorporated into an airbag system 200. Airbag system 200 includes at least one airbag 202 and an inflator 10 containing a gas generant composition 12 in accordance with the present invention, coupled to airbag 202 so as to enable fluid communication with an interior of the airbag. Airbag system 200 may also include (or be in communication with) a crash event sensor 210. Crash event sensor 210 includes a known crash sensor algorithm that signals actuation of airbag system 200 via, for example, activation of airbag inflator 10 in the event of a collision.

Referring again to FIG. 2, airbag system 200 may also be incorporated into a broader, more comprehensive vehicle occupant restraint system 180 including additional elements such as a safety belt assembly 150. FIG. 2 shows a schematic diagram of one exemplary embodiment of such a restraint system. Safety belt assembly 150 includes a safety belt housing 152 and a safety belt 100 extending from housing 152. A safety belt retractor mechanism 154 (for example, a spring-loaded mechanism) may be coupled to an end portion of the belt. In addition, a safety belt pretensioner 156 containing propellant 12 and autoignition 14 may be coupled to belt retractor mechanism 154 to actuate the retractor mechanism in the event of a collision. Typical seat belt retractor mechanisms which may be used in conjunction with the safety belt embodiments of the present invention are described in U.S. Pat. Nos. 5,743,480, 5,553,803, 5,667,161, 5,451,008, 4,558,832 and 4,597,546, incorporated herein by reference. Illustrative examples of typical pretensioners with which the safety belt embodiments of the present invention may be combined are described in U.S. Pat. Nos. 6,505,790 and 6,419,177, incorporated herein by reference.

Safety belt assembly 150 may also include (or be in communication with) a crash event sensor 158 (for example, an inertia sensor or an accelerometer) including a known crash sensor algorithm that signals actuation of belt pretensioner 156 via, for example, activation of a pyrotechnic igniter (not shown) incorporated into the pretensioner. U.S. Pat. Nos. 6,505,790 and 6,419,177, previously incorporated herein by reference, provide illustrative examples of pretensioners actuated in such a manner.

It should be appreciated that safety belt assembly 150, airbag system 200, and more broadly, vehicle occupant protection system 180 exemplify but do not limit gas generating systems contemplated in accordance with the present invention.

Method of Manufacture

The fuels of the present invention may be formed by a water-based method for synthesizing polyvinyltetrazoles as provided below. A water-based process contains the following steps. First, a pre-polymer containing a pendant nitrile or cyano group is provided. Any pre-polymer within cyano or nitrile functionality could be used in the synthesis. Examples include poly(cyanoacrylates), poly(haloacylonitriles) where the halogen can be fluorine, chlorine, bromine, or iodide, poly(crotonitriles), poly(triallyl cyanurates), cellulose cyanoethyl ethers, and poly(methacrylonitriles). Certain copolymers or block copolymers containing nitrile functionality are also contemplated for use as a pre-polymer. These copolymers include poly(butadiene/acrylonitrile)s and poly(styrene/acrylonitrile)s, as well as any other polymer blends of any nitrile containing polymer or oligomer. These polymers and copolymers may be purchased from known manufacturers such as Polysciences, Inc. (www.polysciences.com). A preferred polymeric backbone is a polyethylene chain, although any useful backbone may be employed. The nitrile cannot be part of the backbone and must always be a pendant group. Furthermore, the material may be classified as a pendant-nitrile containing pre-polymer. The pre-polymer may exhibit aromatic character.

The pre-polymer is then reacted with a divalent zinc halide, ZnX₂, in the presence of an azide such as sodium azide, water, and a surfactant, all under pressure at high temperature to form a polymer-bound zinc tetrazole organometallic complex. The polymer intermediate complex is then filtered and washed from the reaction media and then converted to a corresponding tetrazole acid by treatment with concentrated acid. The poly(tetrazole) acid can then be converted into a water soluble salt by reacting it with a suitable base. All steps in the process are carried out in water.

The azide can be any metallic azide such as sodium azide, for example. The surfactant can be essentially any useful surfactant. One exemplary surfactant is ammonium lauryl sulfate, provided by Rhodia, Inc. Other surfactants include various soaps or detergents (e.g. Dial®), phase transfer catalysts such as quaternary ammonium salts (such as tetrabutyl ammonium bromide from Aldrich Chemical Company). The zinc halide can be zinc chloride, zinc bromide, or zinc iodide. The concentrated acid can be hydrogen chloride, hydrogen bromide, nitric acid, sulfuric acid, and organic acids such as acetic acid. Preferably, any acid may be employed that results in a polymer suspension solution having a pH of about 1-3. These acids are available from Aldrich Chemical Company, for example. The base(s) employed may be selected from the group including ammonium hydroxide, hydroxylamine, hydrazine, and any other suitable amine with a pKb compatible to form a stable salt with the polymer. These bases are available from Aldrich Chemical Company, for example.

Process conditions may be described as follows: a 1.0 molar equivalent of polyacrylonitrile (exemplary pre-polymer, Polysciences, Inc.) is reacted with 0.5 molar equivalents of zinc bromide dihydrate (Aldrich), 1.1 equivalents of sodium azide (Aldrich), and 0.0025 molar equivalents of ammonium lauryl sulfate as a 28% wt % solution in water (Rhodia). The final molarity based on the amount of polymer in the water is about 1.0. An exemplary batch uses 47.7 grams of a nitrile-containing pre-polymer, 64.29 grams azide, 117.41 grams zinc, and 2.25 ml surfactant, all constituents mixed in 900 ml of water. The ratios may vary as long as an excess of azide is present and at least 0.5 equivalents of zinc are present. The amount of surfactant may also be varied ranging from no more than 0.1 equivalents and no less than 0.000001 equivalents. The amount of water may vary between a 5.0 molar solution and a 0.1 molar solution. All reagents are mixed in the pressure reactor in no specific order and sealed inside. The contents are stirred and heated to 170 degrees Celsius which will reach a pressure of between 80-100 psi. The mixture is then left to react for a period of about 24-48 hours (preferably 24 hours) and then cooled to room temperature. The milky contents are preferably then filtered in a buchner funnel and washed with an equal volume of water.

Next, the contents are dispersed in about 1.0 to 10.0 liters, and more preferably 3.0 liters, of cold water (ranging from 0-24 degrees Celsius) and rapidly stirred. Enough acid is added to make the pH of the suspension between 1-3 and the mixture is continually stirred for about twenty minutes. The suspension is then filtered again in a buchner funnel using a nylon screen and washed with an equivalent amount of water. This leaves a rubbery wet material in the funnel which is removed and cut into small pieces using standard scissors. This material is then suspended in 1.0 liter of water and excess ammonium hydroxide (Aldrich) is added as the suspension stirs (at least one molar equivalent of ammonium hydroxide is preferably used, and more may be used if desired).

The suspension will slowly dissolve on its own, but heat can be applied to quicken the process, wherein the temperature may range from about 25-100 degrees Celsius. The mixture dissolves and becomes very viscous and then stirring is stopped. The solution is then poured onto a flat metallic sheet and air dried to remove any excess ammonia. After that the material is dried further in an oven to a thin film and then ball-milled to a dust. In essence, after the addition of ammonium hydroxide, the product is completely reacted, and any other subsequent step is just drying and processing the material.

By using a water-based system, the need for expensive and possibly toxic and/or flammable organic solvents is eliminated. The overall cost is therefore dramatically reduced while at the same time safety is enhanced. The intermediate zinc complex is easily filtered from the reaction media and easily converted into the corresponding acid or converted further to the water soluble polyelectrolyte salt.

It will be appreciated that the method of forming a polyvinyltetrazole described immediately above is for exemplary purposes only, and that other methods of forming polyvinylazoles, as also described herein for example, may be employed.

The present description is for illustrative purposes only, and should not be construed to limit the breadth of the present invention in any way. Thus, those skilled in the art will appreciate that various modifications could be made to the presently disclosed embodiments without departing from the scope of the present invention as defined in the appended claims. 

1. A gas generant composition comprising: ammonium poly(c-vinyltetrazole) as a fuel provided at about 5-40% by weight of the total gas generant composition; and an oxidizer selected from the group comprising phase stabilized ammonium nitrate, alkali and alkaline earth metal nitrates and perchlorates, basic transitional metal nitrates, and mixtures thereof, said oxidizer provided at about 60-95% by weight of the total gas generant composition, wherein said oxidizer is provided in an amount resulting in an oxygen balance of −4.0% to +4.0%.
 2. The gas generant composition of claim 1 further comprising: a secondary fuel selected from the group comprising nonmetal salts of tetrazoles and triazoles selected from the group including monoguanidinium salt of 5,6′-Bis-1H-tetrazole (BHT.1GAD), diguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.2GAD), monoaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.1AGAD), diaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.2AGAD), monohydrazinium salt of 5,5′-Bis-1H-tetrazole (BHT-1 HH), dihydrazinium salt of 5,5′-Bis-1H-tetrazole (BHT.2HH), monoammonium salt of 5,5′-bis-1H-tetrazole (BHT.1NH₃), diammonium salt of 5,5′-bis-1H-tetrazole (BHT.2NH₃), mono-3-amino-1,2,4-triazolium salt of 5,5′-bis-1H-tetrazole (BHT.1ATAZ), di-3-amino-1,2,4-triazolium salt of 6,5′-bis-1H-tetrazole (BHT.2ATAZ), diguanidinium salt of 5,5′-Azobis-1H-tetrazole (ABHT.2GAD), monoammonium salt of 3-nitro-1,2,4-triazole (NTA.1NH₃), monoguanidinium salt of 3-nitro-1,2,4-triazole (NTA.1GAD), diammonium salt of dinitrobitriazole (DNBTR.2NH₃), diguanidinium salt of dinitrobitriazole (DNBTR.2GAD), and monoammonium salt of 3,5-dinitro-1,2,4-triazole (DNTR.1 NH₃).
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 6. The gas generant composition of claim 1 further comprising: a secondary fuel selected from the group comprising azoles such as tetrazoles, triazoles, and furazans, such as 5-aminotetrazole; metal salts of azoles such as potassium 5-aminotetrazole; nonmetal salts of azoles such as mono- or di-ammonium salt of 5,5′-bis-1H-tetrazole; nitrate salts of azoles such as 5-aminotetrazole nitrate; nitramine derivatives of azoles such as 5-nitraminotetrazole; metal salts of nitramine derivative of azoles such as dipotassium 5-nitraminotetrazole; nonmetal salts of nitramine derivatives of azoles such as mono- or di-ammonium 5-nitraminotetrazole; and, guanidines such as dicyandiamide; salts of guanidines such as guanidine nitrate; nitro derivatives of guanidines such as nitroguanidine; azoamides such as azodicarbonamide; nitrate salts of azoamides such as azodicarbonamidine dinitrate; and mixtures and combinations thereof.
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